Techniques for Line Narrowing
Techniques for decreasing the bandwidth of the output of lasers are well known. Several such techniques used on excimer lasers are discussed by John F. Reintjes at pages 44-50 in Laser Handbook, Vol. 5, North-Holland Physics Publishing, Elsevier Science Publishers B.V. These techniques include the utilization of gratings, including echelle gratings, for wavelength selection. Use of beam expanding prisms ahead of the grating can increase the effectiveness of the grating.
A prior art technique for decreasing the bandwidth of a KrF excimer laser utilizes a line narrowing module with components shown at 6 in FIG. 1. The resonance cavity of excimer laser 2 is formed by an output coupler 4 (which is a 10 percent partially reflecting mirror) and echelle grating 16. A laser beam 20 (having a cross section of about 3 mm in the horizonal direction and 20 mm in the vertical direction) exits the rear of laser chamber 3. This portion of the beam is expanded in the horizonal direction by prisms 8, 10 and 12 and reflected off mirror 14 onto echelle grating 16. Mirror 14 is pivoted to select the narrow band output for laser 2. Grating 16 is arranged in a Littrow configuration so that the selected narrow band of wavelengths is reflected back off mirror 14 and back through prisms 12, 10 and 8 and into laser chamber 3 for amplification. Light at wavelengths outside the selected narrow band is disbursed so that this disbursed out-of-band light is not reflected back into the laser chamber. Typical KrF lasers operating in a pulse mode may have a gain region about 28 inches in length and produce pulses having a duration of about 15 to 30 ns. Thus, photons within the resonance cavity will make, on the average, about 2 to 4 round trips within the cavity. On each round trip, about 90 percent of the beam exits at the output coupler and about 10 percent is sent back for further amplification and line narrowing. The beam is repeatedly line narrowed as it passes through the line narrowing module.
With this prior art arrangement, the bandwidth of the KrF laser is reduced from its natural bandwidth of about 300 pm (full width half maximum or FWHM) to about 0.8 pm. Some applications of KrF lasers require greater narrowing of the bandwidth. Such applications include, for example, sub 0.25.mu. integrated circuit lithography which requires the bandwidth of 0.4 to 0.6 pm.
What is needed is an improvement to prior art line narrowing units which will provide greater narrowing of the bandwidth of laser beams.